The present invention relates to a high strength steel especially suitable for diffusion chromizing having as quenched mechanical properties corresponding to those of quenched and tempered machine construction steels. In order to increase the lifetime of machine parts against corrosion, many different methods are in use to coat parts with a protective layer. One such method is diffusion chromizing.
The diffusion chromizing is accomplished by holding the part to be coated above the temperature of 900.degree. C. in an atmosphere providing chromium atoms, e.g. some chromium halogenide, usually chromium chloride (CrCl.sub.2). Chromium chloride transfers its chromium atom to the surface of the part to be chromized and receives an iron atom from the surface in the so-called exchange reaction.
A chromium rich zone is thus produced at the surface of the iron, from which chromium diffuses inwards. The chromium potential of the atmosphere is usually between 40-60%. An .alpha.-ferrite zone is then formed at the surface of the part. The thickness of that zone increases at the same rate as chromium diffuses inwards and its content exceeds about 11% (at 1100.degree. C.). This can be seen from the Fe-Cr-phase diagram represented in FIG. 1, where isotherm (1) corresponding to the temperature of 1100.degree. C. is drawn. The curved lines (2, 3) are the calculated phase boundaries of .gamma.- and .alpha.-phases.
The formation of .alpha.-ferrite starts, when the chromium content exceeds the value corresponding to the intersection of the isotherm and the .gamma.-(.gamma.+.alpha.) phase boundary being this value at 1100.degree. C. about 11%. The microstructure is totally .alpha.-ferritic at all temperatures when the chromium content exceeds 13.2%. The thickness of the diffusion chromized zone usually means that part of the zone which contains at least 12% chromium. In this application the zone thickness means that part of the metal that has transformed to .alpha.-ferrite during chromizing. Hence no separate layer is formed on the surface of the part like in electrolytic surface treatments or in molten metal dipping treatments, but the protective zone forms in the metal itself by chromium diffusion from the surface inwards.
Some prior art steels are known aimed for diffusion chromizing (DT 2155439, DT 1159238, SW 346817, US 3,717,444). These steels are usually low-carbon grades alloyed either with strong carbide formers alone (Ti, Zr, Nb, Ta) and having thus low hardenability or they have been alloyed with austenite stabilizing elements like manganese (Mn) (DT 2155439, SW 346817) or nickel (Ni) (US 3,717,444) in order to improve their hardenability. In the composition of these steels it has not, however, been taken into consideration the dependence of the chromizing time on the composition. The diffusion chromizing time varies normally from 4 to 12 hours depending on the zone thickness desired and on the composition of the steel. For economic reasons it is thus important to shorten the treating time. The composition of the steel of the present invention is determined having that objective in mind.